High intensive acoustic waves (sound and/or ultrasound) in gases lead to very high velocities and displacements of the gas molecules. As an example, a sound pressure level (SPL) of 160 dB (at approximately 10 cm from the orifice of the generator) corresponds to a particle velocity of 4.5 m/s and a displacement of 33 μm at 22.000 Hz. In other words, the application of high intensity acoustic waves increases the kinetic energy of the gas molecules significantly.
The large displacements and high kinetic energy of the gas molecules in a gaseous medium due to the application of high intensity acoustic waves will make the gas or air around a solid object oscillate with a high amount of energy. When the oscillating gas or air interacts with a solid object or particles then processes like heat transfer and/or mass transport is enhanced.
Certain high intensity acoustic wave generators use pressurized steam or air or another type of a gaseous medium to generate high intensity acoustic waves. One example is e.g. the ultrasound generator disclosed in European patent EP 1381399 by the same applicant. In this generator, a super-critical (i.e. supersonic) jet stream of steam shoots out from a nozzle and slows down in a resonator producing ultrasound. Generators whose principle of operation is based on auto-oscillations of the supersonic stream when braking in a resonator are referred to as Hartmann generators or to be working according to the well-known Hartmann's Principle. There are different types of the Hartmann generators (e.g. classical, stem-jet, disk-jet, and slot-jet, as well as Laval-nozzle Hartmann generators). Another type of high intensity acoustic wave generator using a pressurized gaseous medium (i.e. gas-jet generators) is a Levavasseur whistle.
In these generators, the generated ultrasound and the gas propagates in a certain directions (that depends on the specific design of the generator) as will be discussed for a given generator in connection with FIG. 1.
A common property of such generators is that the gaseous stream and the acoustic waves are not coinciding.
Traditionally, uses of such acoustic devices have primarily involved using the generated acoustic waves for a given purpose while the gaseous medium was seen more as a by-product without any specific use.
In such traditional acoustic devices steam has not been used as a gaseous driving medium with a specific purpose until recently.
The recent use of a heated gaseous medium like heated steam in a Hartmann generator to generate the ultrasound also greatly enhances a disinfection process of food items as described in European patent EP 1381399 by the same applicant. EP 1381399 discloses the application of heated steam and ultrasound to efficiently kill germs or the like at a surface of a food item without causing damage to the food item. The pressurized steam is forced through an ultrasound generator generating ultrasound in the process. The steam is directed at the food item and the ultrasound enhances the disinfection process by supplying energy.
As indicated and explained in greater detail in FIG. 1, in such various types (Classical, Stem-jet, Disk-jet, Slot-jet, Laval-nozzle etc.) of Hartmann generators, Levavasseur whistles, etc., the gaseous medium may have a general direction indicated by the arrow labeled (A), that is generally directed towards the object to be treated or affected with the gaseous medium while the generated ultrasound may have a general direction indicated by the two arrows labeled (B) that is different than the general direction of the gaseous medium (A). During use, at least a part of the generated ultrasound should be reflected to coincide with at least a part of the gaseous medium after exit from the sound generator in order to benefit from the ultrasound before or at substantially the same time as the steam reaches a treatment or affection zone or region of an object to be affected or treated. The reflection can e.g. be provided by placing reflectors, walls, etc. at appropriate locations and/or simply by the design of the disinfection device.
However, the need for reflection of the ultrasound using conventional reflectors causes some loss in energy and thereby some loss in energy of the process. Furthermore, it is currently not simple how to design and where to place the reflectors needed or how to make the overall design of the disinfection device or ultrasound generator in order to ensure that ultrasound is brought in contact with the steam or gaseous medium at an appropriate reaction or treatment zone with sufficient or optimal energy.